The instant invention relates to an apparatus and method for detecting edges in a signal obtained using a bar code scanner, and in particular a method and apparatus capable of effectively detecting the edges even when the signal is obtained from an optical code located in the Fresnel zone of the scanning device.
Scanning devices used for reading optical indicia have become increasingly important in many different work environments. For example, bar code scanners have been used for inventory control in warehouses, grocery stores, automated machine assembly plants, etc. The instant invention is particularly adapted for use in such bar code readers or other scanners of various types in which the scanner is held stationary while the beam is swept across the optical indicia or code. In such scanners, the illumination from the code is detected, using for example a CCD, to produce analog signals. These analog signals transition in polarity from a relatively high polarity to a relatively low polarity as the illumination changes due to black and white regions in the code or optical indicia. The transitions are then processed into a train of pulses corresponding to the code and the pulses are recognized. More particularly, the pulses will correspond generally to numbers or letter depending upon the protocol of the optical indicia read. One such protocol is the universal product code (UPC), commonly used on grocery and other products.
The detailed design of bar code reading instruments is described in U.S. patent application Ser. No. 08/056,887, filed Apr. 2, 1993 by Chay La, and entitled "Non-Contact Actuated Trigger Apparatus for Bar Code Laser Scanner"; U.S. Pat. No. 5,258,604, which was issued to James Behrens et al. on Nov. 2, 1993, and is entitled "Bar Code Scanner"; U.S. Pat. No. 5,237,161, which was issued to Scott R. Grodevant on Aug. 17, 1993 and is entitled "System for Automatically Reading Symbols, Such as Bar Codes, on Objects Which are Placed in the Detection Zone of a Symbol Reading Unit, Such as a Bar Code Scanner"; U.S. Pat. No. 5,212,371, which was issued to John A. Boles et al. on May 18, 1993 and is entitled "Hand Held Bar Code Scanner with Improved Aiming Means"; U.S. Pat. No. 5,200,597, which was issued to Jay M. Eastman et al. on Apr. 6, 1993 and is entitled "Digitally Controlled System for Scanning and Reading Bar Codes"; and U.S. Pat. No. 5,019,698, which was issued to Jay M. Eastman et al. on May 28, 1991 and is entitled "Bar Code Reading System Having Electrical Power Conservation and Laser Radiation Power Limiting Means." The entire contents of all of the patents and the patent application listed above are incorporated herein by reference. This application and the above-identified patent applications and patents are owned by PSC, Inc. (Webster, N.Y.).
U.S. Pat. No. 5,250,790, which was issued to Boris Melitsky et al. on Oct. 5, 1993, entitled "Hand-Mounted Scanner with Automatic Manual Initiation of Reading Indicia"; U.S. Pat. No. 5,250,792, which was issued to Jerome Swartz et al. on Oct. 5, 1993, entitled "Portable Laser Diode Scanning Head"; and U.S. Pat. No. 4,593,186, which was issued to Jerome Swartz on Jun. 3, 1986, entitled "Portable Laser Scanning System and Scanning Methods," all assigned to Symbol Technologies, Inc. (Bohemia, N.Y.), disclose other conventional scanners.
As the uses for such scanning devices increase, the demand for more versatile scanners has also increased. The scanners must be capable in operating in a variety of different lighting conditions and at a variety of different distances. These demands have placed increasing demands on the recognition apparatus since the signal of the indicia read by the scanner will have increased noise and a more complex structure. One problem which is associated with bar code scanners occurs when the scanner is used in the "Fresnel Zone" (or "near field") of operation.
In bar code scanning, most systems have an aperture through which light is transmitted to the optical code and reflections are received. As a result of the aperture, the light is diffracted and produces interference patterns in the beam. Typically, these patterns will exist in the "near field" (i.e., close to the scanner). The diffraction pattern obtained in the near field is a Fresnel pattern. Optical codes read from the "near field" (also herein referred to as the Fresnel zone) present a unique challenge for the optical code readers due to the intensity profile of the read image.
FIG. 1A depicts the expected intensity I(x) of a signal representing a read bar code as a function of distance x for a transition read in the "far field" (i.e., far away from the scanner). FIG. 1B illustrates the intensity profile for a Fresnel diffraction pattern having a Fresnel number of 2. As can be seen in FIG. 1B, the intensity profile has two lobes with a "dip" at the center. The scanning beam profiles for various Fresnel numbers are illustrated in FIG. 1C. More particularly, FIG. 1C illustrates intensity profiles having a Fresnel number N. Note that larger values of N mean that one is moving closer to the source aperture. The Fresnel number N is calculated according to the formula: EQU N=a.sup.2 /.lambda.d,
where a is the aperture radius, .lambda. is the wavelength and d is the distance from the aperture.
One problem encountered by conventional scanners is extracting (detecting) the edges of the code with a signal which is obtained in the Fresnel zone (i.e., when the scanning is carried out at close range or at contact with the scanner housing). As the above equation illustrates, the scanning distance at which a Fresnel pattern is generated depends on the size of the aperture. In a system where a very large aperture is used to produce a large amount of light on the optical indicia being scanned, the Fresnel zone may extend 10 to 20 inches from the scanner. In other words, the larger the aperture the further the near field will extend and the more complex the intensity profile will be at a given distance (FIG. 1C).
Various techniques have been employed to accurately extract transitions (edges) of the code read by a scanner when the scanner is used near the read optical indicia. One such technique reduces the occurrences of the diffraction pattern by placing a very small aperture in front of the laser light emitted from the scanner in order to shorten the length of the Fresnel zone. One drawback associated with this technique is that less light is provided to the optical indicia or code to be read. Thus, the range of the scanner is decreased since for a given laser power, less light reaches the target.
Other conventional techniques used to extract edges in the detected signal do not adequately allow for the many possible different waveforms which are produced when the scanner is used in the Fresnel zone. One such system uses the first derivative of the detected signal and performs a level detect on the signal of the first derivative. FIGS. 2A and 2B illustrate bar code optical indicia 202a, 202b, the detected signal 204a, 204b for the bar code, the first derivative 206a, 206b of the detected signal and the second derivative 208a, 208b of the detected signal, for scanning operations carried out in the "far field" and the "near field", respectively. As illustrated in FIGS. 2A-2B, the edges of the transitions at times t.sub.1 -t.sub.4 (e.g., white-to-black, black-to-white) occur at the center of the peaks in the first derivative 206a, 206b. In the above described conventional system, the level of the first derivative 206a, 206b is examined to detect a mid-scale on each of the peaks to approximate where the edges are. This technique can only approximate the location of the transitions and may produce inaccurate results where the signal has a Fresnel pattern as illustrated in FIG. 2B, for example.
Other conventional systems have attempted to process the undifferentiated bar code signal 204a, 204b but have still been unable to sufficiently detect the edges of complex Fresnel patterns. Still other systems use a combination of the above techniques. One conventional system uses the first derivative signal compared to a phase shifted first derivative signal. Such a technique is described in U.S. Pat. No. 5,210,397, entitled "Differentiating and Integrating Circuit for Translating Bar Code Signals into Corresponding Pulses."
Each of the above techniques does not adequately provide an edge detection apparatus which will work in both the near field and far field while still having a long range for acceptable scanning performance. It is therefore an object of the instant invention to provide an apparatus and method for accurately detecting the edges of a bar code read by a scanner which overcomes the above drawbacks of conventional techniques.
It is another object of the instant invention to provide a scanning device which includes circuitry which effectively processes a detected signal obtained in the Fresnel zone and accurately detects transitions (edges) in this zone.